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Solipsism is the theory that the external world exists only through conciousness. The Party takes advantage of this world view because if reality is truly internal, and conciousness is alterable....... reality itself is a simple matter to modify to one's liking.

"Winston shrank back upon the bed. Whatever he said, the swift answer crushed him like a bludgeon. And yet he knew, he knew, that he was in the right. The belief that nothing exists outside your own mind -- surely there must be some way of demonstrating that it was false? Had it not been exposed long ago as a fallacy? There was even a name for it, which he had forgotten."